With his presidential effort, O'Rourke is hoping to leverage the fame he gained with his Senate race.

He was a heavy underdog when he challenged Republican Ted Cruz in mostly conservative Texas, but he quickly demonstrated an ability to draw large crowds and raise money from voters nationwide.

Beto O'Rourke after the 2018 midterms.

Getty

His Senate bid generated a torrent of media attention and excited voters in a party desperate for fresh political faces. He lost the race by less than three percentage points, the tightest US Senate contest in the state in four decades.

Early opinion polls on the 2020 race have consistently ranked O'Rourke in the top tier of contenders, behind former Vice President Joe Biden, who has not yet said whether he is running, and US Senator Bernie Sanders of Vermont - a 2016 presidential contender who announced his 2020 candidacy last month.

Since his Senate bid ended, O'Rourke has worked to keep himself in the public eye, regularly staying in touch with his supporters and sitting for an interview with Oprah Winfrey.